The measurement of pressure in pipelines and other operating fluid chambers is very important to many industrial applications, and in particular to gas, petroleum, sewage and water utilities. Irregular pressures can cause catastrophic effects to mechanical systems and result in large losses of time and money.
Generally, pressure pipelines are designed with enough structural strength to withstand both normal operating pressures and transient pressures. Pressure transients occur whenever there is a change in the flow rate in a pipeline and can be significantly higher and/or lower than normal operating pressures. Causes of transient pressures include opening or closing a valve, starting or stopping a pump, or operation of an air relief valve.
Under normal circumstances, transient pressures are predictable and readily accommodated by pipeline design. For example, main line butterfly valves are designed to close over a period of minutes to minimize transient pressures. Pump motors are designed to start against a closed valve and the valve gradually opens to minimize transient pressures.
The presence of air pockets has a number of potentially adverse effects on the operation of a pipeline, including the aggravation of transient pressures. Therefore, air valves are included in pipeline design to discharge accumulated air pockets to minimize this problem.
Other instances of transient pressures are more difficult to predict accurately and, thus, they are not included in pipeline design. For example, a sudden power outage in a pumped pipeline system causes an abrupt cessation of flow in the pipeline and a large transient pressure. This is a predictable transient, although it is very difficult to analyze and design a system to deal with this type of transient. In a worse case scenario, a power loss causes cavitation in the water column and an extremely high pressure over a short duration. The presence of air pockets in the pipeline aggravates this problem by increasing the chances of cavitation, water column separation and damaging pressures. Water column separation results with the appearance of negative pressures in certain reaches of a water main. Pressures drop to water vapor pressure, causing vapor pockets. When the inertia of the water column is overcome, the direction of flow reverses, causing the vapor pockets to collapse and the separated columns to rejoin. Extremely high, destructive pressures result.
Another example of problematic transients is the rupture of a pipeline causing flow rates far in excess of design velocities. Attempts to close butterfly or similar valves can result in catastrophic structural failure of the valve. Pressures of this magnitude are not anticipated by pipeline design.
Hydraulic transient analysis procedures do exist, however, transient pressure prediction is a complex procedure requiring digital modeling of specific pipeline configurations, operating procedures and expected flow considerations. Considerable judgment and experience is needed to model a pipeline operation and accurately anticipate those conditions that will result in the highest transient pressures. Frequently, pipeline design simply predicts that transient pressures will be a fixed percentage above normal operating pressure and pipelines are then designed on this assumption. For example, transient pressures in water pipelines may be assumed to be 40% above normal operating pressure.
Damage from transient pressures can be benign or catastrophic. Less serious effects include gradual spalling of the inner surface of the pipe or damage to joint materials. In concrete pressure pipe materials, the stress levels may result in cracking of mortar on the exterior surface of the pipe, leading to the eventual compromise of the protection of prestressing. This damage, in turn, results in the introduction of water and air to the steel and subsequent corrosion. The corrosion, gradual fracture and deterioration can lead to catastrophic rupture many years after the damaging events. When rupture does occur, there will be no record of the source of the problem. Alternatively, the most severe transient events may cause movement of a pipe or an immediate catastrophic rupture. Damage is most severe in thin-walled pipes, lined pipes and concrete cylinder pipes.
Most of the country's infrastructure is aging and there are limited funds for replacement. Unpredictable pressure transients can have a severe effect on these systems. The resultant distress from transient pressures accumulates over time, causing a rupture long after the damaging transient occurs.
Current systems for detection of transient pressures are not adequate to measure and record severe transient pressures. Current analog pressure measurement systems continuously record pressure at a constant rate. This rate is established to present the data in the timeframe and format required by the user, but the fixed rate does not have the flexibility to present detailed data concerning sharp transient pressures when these transient pressures are detected. Current digital pressure measurement systems measure and record pressure data at a predetermined, fixed interval. The interval may be set permanently into the system, or it may be user adjustable. For instance, the interval may be once per day, once per hour, or even once per minute in the most rigorous pressure measurement systems. However, some of the most severe transients will have a duration of less than one second, and will not be accurately measured by set-interval data recording systems. Existing systems cannot, in a practical way, measure and record the most severe, unpredictable transients.
Needs exist for improved and practical methods for detecting and accurately recording transient pressures in pipelines and other operating fluid chambers.